World's oldest digital computer restored to former glory

The Harwell Dekatron, or WITCH, is now ready for use at the National Museum of Computing at Bletchley Park, following a three-year restoration project.

The National Museum of Computing (TNMOC) at Bletchley park has announced the switching on of the world's oldest working digital computer, the Harwell Dekatron, following the successful completion of a three-year restoration project.

Built in 1949 by the Atomic Energy Research Establishment in Harwell, Oxfordshire and first used in 1951, the Dekatron is the oldest surviving digital computer. Constructed from 828 Dekatron valves and 131 other valves, 480 physical relays, a total of 7,073 contacts, 199 lamps, 18 switches, and a bank of paper tape readers, its 2.5 tonne chassis is barely recognisable as a computer. Even its method of operation is unique in the modern world: unlike today's computer systems, which operate in binary, the Harwell Dekatron operates in base 10 - the same decimal system as adopted by most human societies, in deference to our ten fingers.

Designed to automate atomic energy calculations previously worked out laboriously by hand, with the staff who operated the hand-cranked mechanical calculators enjoying the job title of 'computer,' the Harwell Dekatron was designed with reliability in mind. Not the fastest system in the world, it was nevertheless capable of running flat-out for days - something rare to see in the days of unreliable valves and large, hot-running components.

By 1957, the Harwell Dekatron had become too slow for the calculations being made by the Atomic Energy Research Establishment. Rather than scrap the device, a fate to which that all too many irreplaceable systems from throughout the history of computing were resigned, one scientist suggested the system be offered as-is for educational use. Having put forward the best use case Wolverhampton and Staffordshire Technical College were the lucky recipients of the machine, rebranding the system to the Woverhampton Instrument for Teaching Computation from Harwell, or WITCH.

As the Harwell WITCH, the Dekatron would continue to be used right through to 1973 as a means of teaching digital computation. As binary computers became increasingly popular, however, its ability to prepare students for real-world computation decreased, and the system was retired in 1973. The Harwell WITCH was displayed as a non-working artefact at the former Birmingham Museum of Science, but the closure of the museum in 1997 combined with the WITCH's unwieldy size led to it being dismantled and being placed into storage.

Fast-forward to 2008, when the Harwell WITCH was rediscovered by a team of volunteers from the National Museum of Computing. 'I first encountered the Harwell Dekatron as a teenager in the 1970s when it was on display in the Birmingham Museum of Science and Industry - and I was captivated by it,' recalls Kevin Murrell, TNMOC trustee and initiator of the restoration project. 'When that museum closed, it disappeared from public view, but four years ago quite by chance I caught a glimpse of its control panel in a photograph of stored equipment. That sparked our ideas to rescue it and we hunted it down.

'The TNMOC restoration team has done a superb job to get it working again and it is already proving to be a fascination to young and old alike. To see it in action is to watch the inner workings of a computer - something that is impossible on the machines of today. The restoration has been in full public view and even before it was working again the interest from the public was enormous.'

The Harwell WITCH, in its fully-functioning form, is now on display at the National Museum of Computing at Bletchley Park near Milton Keynes, where it will serve as a tool to teach computational thinking and computer history as well as a contrast to the previously-rebuilt Colossus system, the world's first semi-programmable electronic computer, which can also be seen at the museum.

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8 Comments

....i want to say "but can it...." but i'm holding back as this is a legendary computer, up their with other legendary computers and digital devuces such as the casio watch of the 1980's, the Amiga 1200 and Stephen hawkings.

"Even its method of operation is unique in the modern world: unlike today's computer systems, which operate in binary, the Harwell Dekatron operates in base 10 - the same decimal system as adopted by most human societies, in deference to our ten fingers."

It makes me wonder how exactly this operated. The thing about modern digital computers and binary is binary is a complement to digital, where something is either on or off, 0 or 1, yes or no. The only way I can really understand how the Dekatron works is if this computer only has a handful of specific built-in tasks (in other words, can not be programmed to do anything else). I can't really imagine how a programmable computer can be operated in decimal.

Originally Posted by Griffterit does not say anything to put it into perspective how fast or rather slow it is in todays standard pc's... like it took it 5min to calculate the e=MC2 or whatever. can someone give a reference?

It takes about five minutes to do a long multiplication - roughly the same length of time it takes someone to do the same sum by hand on a comptometer. Its design goal was reliability, not speed: although it was no faster than a human, it could keep running for days. There's a story from its early days: one of the scientists challenged the system to a race. Being fast with his figures, he pulled out into an early lead - but had to stop, exhausted, after half an hour. The Dekatron, meanwhile, kept on going. Its longest uninterrupted run was around ten days - and while that might not seem like much when I'm working on a desktop that had been running for three weeks before I took it down for a kernel upgrade, in the days of relays and valves that was something pretty special.

So, you're looking at a machine that takes five minutes to do something a 3GHz desktop could do in a few millionths of a second.

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